There have been increased demands for fuel saving of automobiles as part of the energy conservation policy in recent years. According to such demands, torque reduction in bearings used in automobiles has been required. Furthermore, as to a bearing supporting a transmission, a differential and the like of the automobile, the space for housing such a bearing has tended to be reduced due to multistage transmissions and increased space for driving operations. Accordingly, the load applied per size of the bearing has been increased. Furthermore, the bearing used in an automobile has also been required to have an increased life under the oil lubrication environment in which foreign substances are mixed. Furthermore, it is desirable to manufacture such a bearing using an inexpensive material in a low cost manner. Thus, it is desirable to manufacture a bearing using a material that can be available in every country in the world by a simple method that does not lead to an increase in cost.
Examples of a bearing that supports a transmission, a differential and the like in an automobile may be a tapered roller bearing. The tapered roller bearing is formed of: an inner ring in which a small collar and a large collar are provided on both sides of a raceway surface formed on an outer circumferential surface; an outer ring in which a raceway surface is formed on an inner circumferential surface; a plurality of tapered rollers arranged between the raceway surfaces of the inner ring and the outer ring; and a holder holding the tapered roller housed in a pocket. Furthermore, the holder is formed of: a small annular portion continuously extending on the side of a small diameter end face of the tapered roller; a large annular portion continuously extending on the side of a large diameter end face of the tapered roller; and a plurality of column portions that couple these annular portions. Furthermore, the pocket is formed in a planar shape having a trapezoidal shape in which a portion housing the small diameter side of the tapered roller is located on a reduced width side while a portion housing the large diameter side is located on an increased width side.
In a transmission, a differential and the like of the automobile, the tapered roller bearing is used in the state where its lower portion is immersed in an oil bath, and therefore, the oil in the oil bath flows into the bearing as a lubricating oil in accordance with rotation of the tapered roller bearing. In such an application, the lubricating oil flows into the bearing from the small diameter side of the tapered roller. Then, the lubricating oil flowing from the outer diameter side relative to the holder flows into the large diameter side of the tapered roller along the raceway surface of the outer ring. Also, the lubricating oil flowing from the inner diameter side relative to the holder flows into the large diameter side of the tapered roller along the raceway surface of the inner ring.
In this way, as a tapered roller bearing used in the situation where lubricating oil flows in from the outside, there is disclosed a tapered roller bearing configured to have a holder having a pocket provided with a cutout, so that the lubricating oil divided and flowing into the outer diameter side and the inner diameter side of the holder is caused to flow through the cutout, thereby improving circulation of the lubricating oil through the bearing (for example, see Japanese Patent Laying-Open No. 09-32858 (PTD 1) and Japanese Patent Laying-Open No. 11-201149 (PTD 2)).
Furthermore, according to the tapered roller bearing into which lubricating oil flows into the bearing while being divided into the outer diameter side and the inner diameter side of the holder, the proportion of the lubricating oil flowing from the inner diameter side of the holder into the inner ring side is increased, which causes a problem that torque loss increases. In other words, the lubricating oil flowing from the outer diameter side of the holder into the outer ring side smoothly passes along the raceway surface of the outer ring toward the large diameter side of the tapered roller and flows out from the inside of the bearing. In contrast, the lubricating oil flowing from the inner diameter side of the holder into the inner ring side is blocked by a large collar formed on the outer circumferential surface of the inner ring and retained within the bearing. Accordingly, when the proportion of the lubricating oil that flows from the inner diameter side of the holder into the inner ring side is increased, the amount of the lubricating oil retained within the bearing increases, which causes a problem that this lubricating oil results in a flow resistance against rotation of the bearing, thereby increasing torque loss. For addressing this problem, there is disclosed a tapered roller bearing configured to have a holder having a trapezoidal-shaped pocket that is provided at its column portion on the reduced width side with a cutout, so that the lubricating oil flowing from the inner diameter side of the holder into the inner ring side is allowed to escape toward the outer ring side, thereby allowing a reduction in the amount of the lubricating oil retained within the bearing (for example, see Japanese Patent Laying-Open No. 2007-24168 (PTD 3)).